The invention relates generally to steam turbines and, more particularly, to an inner shell assembly for a steam turbine including common grooves to facilitate inner shell manufacture.
A steam turbine is a mechanical device that extracts energy from pressurized steam and converts the energy into useful work. Steam turbines receive a steam flow at an inlet pressure through multiple stationary nozzles that direct the steam flow against buckets rotationally attached to a rotor of the turbine. The steam flow impinging on the buckets creates a torque that causes the rotor of the turbine to rotate, thereby creating a useful source of power for turning an electrical generator or other mechanical device. The steam turbine includes, along the length of the rotor, multiple pairs of nozzles (or fixed blades) and buckets. Each pair of nozzle and bucket is called a stage. Each stage extracts a certain amount of energy from the steam flow causing the steam pressure and temperature to drop and the specific volume of the steam flow to expand. Consequently, the size of the nozzles and the buckets (stages) and their distance from the rotor grow progressively larger in the later stages.
Steam turbine customers require unique steam turbine designs that are optimized for the customer's plant and yield economically appropriate delivery, cost, performance, reliability, availability, and maintainability. Historically, this customer need has been met by supplying steam turbine steam paths that are unique to the customer's plant. In the past, the inner shells, carriers, and other components were designed specifically for each steam path. This approach led to longer design and procurement cycles for large components such as the shells and inner casings, the proliferation of shell and inner casing designs, and the inability to inventory common or spare components to support customer demand.
FIG. 1 shows prior art inner shell grooving design for wheel and diaphragm type construction. A single shell section with nine stages is shown. The nozzle carrier (diaphragm) for each stage is supported in an individual groove custom machined on the inner surface of the shell. The diameter of the shell groove is established based on the tip diameter of the stage's bucket. The use of this shell for steam paths with larger tip diameters or more stages is extremely limited with this design. This design provides centerline support and alignment provisions for each nozzle of each stage.
FIG. 2 shows prior art for a shell/carrier grooving design for carrier type construction. FIG. 2 shows a section with one shell, two carriers, and 27 reaction stages. There are two nozzle carriers that support the nozzles of their respective stages. Each nozzle is supported in an individual groove machined on the inner surface of their respective carrier. The diameter of the carrier groove is established based on the tip diameter of the stage's bucket. The use of carriers for steam paths with larger tip diameters or more stages is extremely limited with this design. Stage alignment with this design is limited to carrier alignment capability (individual stage alignment is not possible). For this design, average alignment for stages 1-16 and stages 17-27 is feasible.
It would be desirable to provide a modular, flexible, common steam turbine shell/inner casing design that will accommodate a wide range of steam paths. Such structure would serve to reduce the need to provide multiple designs for steam turbine shell/inner casings designs and provide for a dramatic decrease in the time needed to design and procure steam turbine shells/inner casings. Additionally, such structure would facilitate the ability to carry shell and inner casing inventory to further expedite the turbine delivery cycle.